In the field of adhesives, polyurethane-based reactive single-component formulations are often employed because of their following advantageous properties:
good mechanical strength of the bonded assembly, PA1 adhesion to substrates of various natures, PA1 improved resistance to hydrolysis when compared with traditional adhesives,in particular hot-melt adhesives, PA1 compatibility with treatments subsequent to the bonding operation (for example painting), PA1 good chemical resistance, and PA1 good heat strength. PA1 1) 5 to 100 meq. OH of an ethylenically unsaturated monomer containing at least one hydroxyl group per mole, PA1 2) 60 to 90% by weight of ethylene, PA1 3) 10 to 40% by weight of vinyl acetate. The percentage being calculated relative to the total weight of ethylene and vinyl acetate PA1 1) 10 to 50 m eq. OH of an ethylenically unsaturated monomer containing at least one hydroxyl group per mole, PA1 2) 65 to 80% by weight of ethylene, PA1 3) 20 to 35% by weight of vinyl acetate. PA1 R.sub.2 denotes H or a C.sub.1 -C.sub.4 hydrocarbon radical, PA1 R.sub.3 denotes CO--O--(CH.sub.2).sub.x, x being between 1 and 5, or (CH.sub.2).sub.n, n being between 0 and 10 inclusive.
These properties result from the presence of free isocyanate groups in the polyurethane which, after processing, react with moisture and/or the reactive functional groups of the substrates to be assembled, causing a subsequent partial crosslinking.
However, the polyurethane-based formulations which have just been mentioned exhibit, as a major disadvantage, a low rate of rise in cohesion. As a result it is necessary to hold the bonded components for a sufficient time to enable the assembly to be handled, which is found to be prohibitive when the intention is to produce adhesive bondings with machines operating at a high output rate.
Various solutions have been proposed in order to improve the rate of rise in cohesion immediately after the application.
Thus in EP-A-107 097 it has been proposed to add a polyurethane or a thermoplastic crystalline polyester to the hot-melt compositions based on an isocyanate prepolymer and on a synthetic resin of low molecular weight.
Hot-melt adhesives have also been described which include a urethane prepolymer and polymers of low molecular weight (see EP-A-246 473) or a thermoplastic polymer of ethylene and of a vinyl monomer and a tackifying agent (see EP-A-289 945).
In EP-A-402 854 a hot-melt composition is formed by reacting a polyisocyanate and a copolymer of ethylene, of vinyl acetate and of ethylenically unsaturated monomers containing hydroxyl functional groups.
Besides the low rate of rise in cohesion, the adhesives which have just been mentioned exhibit the disadvantage of being applicable only at an elevated temperature (in the case of crystalline polyesters) or of exhibiting a plasticization of the adhesive after crosslinking (in the case of thermoplastic polymers).